This invention relates to printed circuits, and more particularly to compositions for producing dielectric layers for use in such circuits.
It is useful in fabricating printed circuits to be able to conserve space by disposing a metallization directly above other metallizations. To prevent shorting and reduce capacitance coupling, such metallizations are separated by dielectric material.
There are two ways to produce such multilayer structures. The first consists of printing and firing "crossover" layers between printed conductor layers on a single substrate, to form what is sometimes called a "multilevel" printed wiring board. The second method involves printing conductor patterns on organic-bonded thin "tapes" of particulate alumina, then laminating such printed tapes and firing the resultant laminated structure at high temperature to make a discrete monolithic multilayer structure which serves as its own substrate. The present invention describes the role of certain compositions in forming, inter alia, crossover dielectric layers in the "multilevel" type of process, wherein the substrate is a prefired alumina ceramic.
A crossover dielectric composition is essentially a low dielectric constant insulator capable of separating two conductor patterns through several firing steps. High melting, viscous glasses have been used as the dielectric so that the firing of the top conductor line can be carried out at a temperature below that at which softening of the dielectric occurs. Melting or softening of the crossover dielectric is accompanied by shorting of the two conductor patterns against each other with subsequent failure of the electrical circuit. The major requirement for a crossover dielectric is control of resoftening or thermoplasticity in the top conductor firing step. Other property requirements are: (a) low dielectric constant to produce low A.C. capacitance coupling between the circuits insulated by the crossover dielectric, (b) low electric loss (high Q) to avoid dielectric heating, (c) low "pinholing" tendency and a low tendency to evolve gasses in firing, (d) proper glass softening temperature so that the initial firing is adaptable to the screen printing process, (e) a high resistance to thermal shock crazing, and (f) low sensitivity to water vapor and subsequent spurious electrical losses.
Also required are compositions for producing dielectric layers in multilayer capacitors printed on an alumina substrate. Such capacitors include those of Bacher et al. U.S. Pat. No. 3,683,245 and Bergmann U.S. Pat. No. 3,679,943, each of which is incorporated by reference herein.
Among the numerous compositions known for producing dielectric layers in multilayer structures are compositions based upon glasses, such as the crystallizable glasses of Hoffman U.S. Pat. No. 3,586,522 or Amin U.S. Pat. No. 3,785,837; or upon mixtures of crystalline materials and glasses such as Amin U.S. Pat. No. 3,787,219 and Bacher et al. U.S. Pat. No. 3,837,869. Each of these four patents is incorporated by reference herein.
Often the alumina substrate on which multilayer structures are formed is distorted or bowed by forces exerted by the fired dielectric layer(s). There is a need for dielectric compositions which have thermal expansion characteristics such that bowing is reduced, since otherwise poor film adhesion can result.
Reduction in alumina substrate bowing caused by many commercially available dielectric compositions is important, since distorted (non-planar) substrates makes allignment difficult in printing subsequent layers on the substrate. Also, bowed substrates are more difficult to mount into connector assemblies. Furthermore, the compressive forces exerted by the dielectric layer can result in cracking of the alumina substrate when it undergoes thermal cycling, for example, in dip soldering of the electrodes. The fired dielectric layers must be nonporous, as defined herein, and fireable at temperatures compatible with the firing temperatures of typical electrode compositions (e.g., below 975.degree. C.). Furthermore, when crystalline fillers are used, the fillers should have dielectric constants which are relatively low.